Method of producing homogeneous ingots of a metallic alloy

ABSTRACT

A method for producing homogeneous ingots of an alloy in which one of the components thereof has a high vapor pressure includes the step of sealing the desired alloy components in a quartz ampoule sufficiently large to include a vapor space therein and heating the ampoule and its contents to at least a temperature at which the components will form a homogeneous liquid mixture. An example of an alloy which can be produced by the instant process is an alloy of cadmium, mercury and tellurium. The method of this invention also includes the steps of quenching the homogeneous alloy from the liquid to the solid state while maintaining a relatively high vapor pressure in the vapor space in the ampoule above the space occupied by the alloy. The high vapor pressure can be maintained in the vapor space by thermally insulating the vapor space during the quenching step. The preferable form of quenching is by immersing only a portion of the space occupied by the alloy in a quenching bath, for example, a quenching oil.

United States Patent [151 3,656,944 [451 Apr. 18, 1972 Brau [54] METHOD OF PRODUCING HOMOGENEOUS INGOTS OF A METALLIC ALLOY [72] Inventor: Maurice J. Brau, Richardson, Tex. [73] Assignee: Texas Instruments Incorporated, Dalla Tex.

[22] Filed: Feb. 16, 1970 [211 App]. No.: 11,644

[52] US. Cl. ..75/135, 75/134 H, 75/169, 75/ 151 [51] Int. Cl ..C22c 1/00 [58] Field oiSearch ..75/135, 134 H, 151,169;

252/62.3 R, 62.3 ZT, 501; 23/315 [56] References Cited UNITED STATES PATENTS 3,468,363 9/1969 Parker et al. ..164/125 2,953,690 9/1960 Lawson et al ..250/211 3,318,669 5/1967 Folberth ..23/315 3,335,084 8/1967 Hall ..252/62.3

Primary Examiner-Dewayne Rutledge Assistant Examiner-E. L. Weise Att0rneySamuel M. Mims, Jr., James 0. Dixon, Andrew M. Hassell, Harold Levine, Melvin Sharp, John E. Vandigriff and William E. Hiller [5 7] ABSTRACT A method for producing homogeneous ingots of an alloy in which one of the components thereof has a high vapor pressure includes the step of sealing the desired'alloy components in a quartz ampoule sufficiently large to include a vapor space therein and heating the ampoule and its contents to at least a temperature at which the components will form a homogeneous liquid mixture. An example of an alloy which can be produced by the instant process is an alloy of cadmium, mercury and tellurium. The method of this invention also includes the steps of quenching the homogeneous alloy from the liquid to the solid state while maintaining a relatively high vapor pressure in the vapor space in the ampoule above the space occupied by the alloy. The high vapor pressure can be maintained in the vapor space by thermally insulating the vapor space during the quenching step. The preferable form of quenching is by immersing only a portion of the space occupied by the alloy in a quenching bath, for example, a quenching oil.

10 Claims, 2 Drawing Figures METHOD OF PRODUCING HOMOGENEOUS INGOTS OF A METALLIC ALLOY v This invention relates to the production of alloys and more specifically to the production of homogeneous ingots of an alloy in which one of the components thereof has a relatively high vapor pressure.

Specialized alloys, specifically alloys of mercury, cadmium and tellurium have been difficult to produce in appreciable quantities usable for a specified purpose. A conventional technique for preparing an alloy such as mercury cadmium telluride is to seal the proper portions of the elements in a thick-walled quartz ampoule, which is then evacuated and sealed. The ampoule is placed in a conventional rocking furnace and reacted at approximately 800 C. for twenty-four hours. The ampoule is removed from the rocking furnace, cooled to room temperature and loaded into a conventional vertical Bridgman furnace where it is dropped through a steep temperature gradient (approximately 200 C. per centimeter) at the rate of 2 to 3.2 millimeters per hour. The resulting ingot of mercury cadmium telluride may contain one or several single crystal regions. The slow freezing which the mercury cadmium telluride experiences in being dropped through the temperature gradient of the Bridgman furnace, within which gradient the mercury cadmium telluride will be transformed from the liquid to the solid state, results in a mercury cadmium tellurium composition different from that of the originally formulated composition. In other words, a concentration gradient will be established from one end of the ingot to the other due to segregation of the component atoms during the slow freezing process. There is not only a longitudinal concentration gradient, but also a radial concentration gradient in the ingot produced by the forgoing method. Thus, if a desired composition of, for example, 40% mercury, 10% cadmium and 50% tellurium is desired, it is necessary to take the cross sections through the ingot at different points along the length, analyze each of these sections, and utilize only the section having the desired concentration. This technique of producing a mercury cadmium telluride alloy results in a yield of on the average around 10% of the originally formulated composition.

Another method for producing ingots of mercury cadmium telluride is disclosed in US. Pat. No. 3,468,363 (TI-2759). In this process, the desired proportions of mercury, cadmium and tellurium are formulated and sealed in a thick-walled quartz ampoule. Like the foregoing prior art process, the ampoule is then placed in a conventional rocking furnace and reacted at 800 C. for approximately 24 hours. In this process, however, the ampoule is radially surrounded by quartz wool. The ends of the rocking furnace are also plugged with quartz wool. After the reaction period has expired, the end of the ampoule containing the molten alloy is contacted with a stream of cool gas, for example, nitrogen. The gas stream cools the liquid alloy to a solid state at a relatively rapid rate. However, since the cooling is rapid, the final ingot contains some blow holes or cavities and is piped" or contains piping throughout a portion of the ingot. This process represents an improvement over the previous processes, but still results in a usable yield of around 50 percent of the originally formulated composition.

Another method for eliminating the problem of Bridgman furnace technique described above has been suggested. For example, it has been proposed to remove the ampoule from the rocking furnace and immediately drop it into a water or ice bath to effect immediate transition of the mercury cadmium telluride from the liquid to the solid state. However, as pointed out in US. Pat. No. 3,468,363, this technique produces an ingot having from moderate to severe piping or pitting in the core of the ingot. This piping is believed to be caused by mercury vapor erupting from the liquid into the vapor space of the liquid, as the vapor space above the liquid cools more quickly than the liquid phase causing an imbalance in the equilibrium within the ampoule. This imbalance tends to cause the mercury vapor to move from the hotter liquid phase into the cooler vapor phase where it condenses. In fact, close inspection of such an ingot produced by the quick freezing technique will reveal liquid mercury dispersed within the pits or cavities formed in the ingot by the quick freezing process. The resulting pitted alloy ingot is worthless, since to be usable for a specified purpose, slices of the ingot cannot contain discontinuities.

It is therefore desirable to possess a method or process by which alloy ingots, specifically ingots composed of mercury, cadmium and tellurium, can be produced such that nearly all of the originally formulated mixture results in usable alloy of the desired composition. It is also desirable to possess a method by which molten alloys can be quickly solidified without producing blow holes piping throughout portions of the ingot. This invention, therefore, provides a method of producing an alloy in which one of the components has a relatively high vapor pressure by sealing the desired alloy components in an ampoule sufficiently large to include a vapor space therein and heating the ampoule and contents to at least the temperature at which the components will form a homogeneous liquid mixture, the improvement in this method comprising quenching the homogeneous alloy from the liquid to the solid state while maintaining a high vapor pressure in the vapor space above the space occupied by the alloy. More specifically, in a method of producing a mercury cadmium telluride alloy by sealing the desired proportions of mercury, cadmium and tellurium in an ampoule sufficiently large to include a vapor space and heating the ampoule and contents in a furnace to at least a temperature at which the components will form a homogeneous liquid mixture, the improvement comprising therrnally insulting the portion of the ampoule containing the vapor space, removing the ampoule from the furnace, and quenching the alloy from the liquid to the solid state by cooling at least a portion of the space occupied by the alloy. Quenching is preferably accomplished by immersion in an oil quenching bath.

A better understanding of the invention will be derived by reference to the ensuing specification and the accompanying drawings in which:

FIG. 1 is a schematic cross sectional view of a rocking furnace; and

FIG. 2 is a schematic view of an ampoule and contents being cooled in a quenching bath; and

FIG. 3 is a schematic cross sectional view of an alternate embodiment to that shown in FIG. 2.

Although the invention will be described in relation to a preferred embodiment of producing a mercury cadmium telluride alloy, it is intended that the invention be limited only as defined in the appended claims. Mercury cadmium telluride is a ternary composition which is often described as a pseudo-binary since the mercury and cadmium behave as though they were only one element in combination with tellurium. This composition is highly useful for the manufacture of electromagnetic radiation detectors. By varying the composition of the material, the detectable wavelength of the devices fabricated therefrom can be varied over a considerable range. It is desirable to pick a given wavelength to be detected and tailor the mercury cadmium telluride alloy such that it is capable of detecting the desired wavelength. Thus, the exact composition of the material becomes very important.

Referring to FIG. 1, a conventional electrically powered resistance-heated rocking furnace 10 is composed of an alumina wall 12 which defines a cylindrical chamber 14. Positioned within the chamber 14 is a quartz ampoule 16 containing a mixture 18 of the desired proportions of mercury, cadmium and tellurium. The mixture is shown in a liquid state. The portion of the ampoule which is not occupied by liquid, i.e., the vapor space 20, is surrounded by a thermal insulator 22. The insulator can be composed of microquartz or can be composed of a fire brick plug as shown. A heat sink, such as stainless steel, could also be employed to maintain a suitable temperature in the void space. The insulator 22 has a cavity 24 into which the portion of the ampoule containing the vapor space is inserted. The top end of the furnace 10 has a plug 28 of quartz wool tightly packed therein to retain heat within the chamber 14. The lower end of the furnace has a plug 30 of quartz wool packed therein surrounding a stainless steel conduit 32, one end of which opens toward the bottom of ampoule 16.

In operation, the ampoule 16 is inserted into the cavity 24 of fire brick insulator 22. The ampoule and insulator are inserted into the cavity 14 of the furnace 10. The end plugs 28 and 30 are then inserted and the steel conduit 32, which is at the same time positioned to direct a flow of gas toward the bottom of the ampoule 16. After the molten alloy or mixture 18 has been heated at a temperature above its melting point for a sufficient length of time, power to the heating element is cut off and a stream of cool gas, for example, nitrogen, is caused to flow from end 34 of conduit 32 and impinge upon the bottom of the ampoule 16. The molten alloy 18 is cooled from the liquid state to the solid state while retaining sufficient heat in vapor space to maintain therein a relatively high vapor pressure of at least one of the components of the molten alloy. For the mercury cadmium telluride alloy, the vapor pressure of mercury is the significant factor. After the alloy 18 has been solidified, the ampoule 16 can be removed from the furnace and thereafter further processed.

Referring to FIG. 2, a second and preferred embodiment of the present invention is illustrated. In this embodiment, the ampoule 40 is partially filled with alloy 42 and has a vapor space 44 surrounded and thermally insulated by a fire brick or other suitable insulator 46. The ampoule and contents are heated in a rocking furnace by a procedure similar to that described in conjunction with FIG. 1. However, no tube for cooling gas is necessary. For ease of handling, an eyelet 48 has connected thereto a wire 50 extending through a channel 52 in the insulator 46. After the ampoule, its contents and the insulator have been heated at a temperature sufficient to maintain the alloy in a liquid state for the desired amount of time, the ampoule 40 and insulator 46 are together removed from the rocking furnace and immediately thereafter quenched. In the preferred embodiment, at least a portion of the ampoule 40 containing the alloy 42 is immersed in a quenching bath 54 in a suitable container 56. Most preferably, only a portion of the ampoule 40 containing alloy 42 is immersed in the bath 54. The quenching bath 54 can be any suitable quenching liquid; it is preferably a conventional metallurgical quenching oil.

The function of the insulator in both embodiments, FIG. 1 and FIG. 2, is to retain heat in the vapor space 20 (FIG. 1) and 44 (FIG. 2) to prevent rapid cooling of the vapor therein. Thus, in the case of the alloy mercury cadmium telluride, the vapor pressure of mercury is maintained at a relatively high level while cooling the molten alloy. So maintaining the vapor pressure in the vapor space 44 prevents flashing of mercury from within the molten alloy body while the latter is cooling. This, of course, prevents the production of blow holes and prevents piping or pitting in the solidified alloy ingot. Additionally or alternatively, as shown in FIG. 3, an auxiliary electric resistance heater 58 insulated by a layer 50 of quartz wool can be positioned about the vapor space to maintain a higher temperature and thus a higher vapor pressure in the vapor space.

For a more complete understanding of the present invention and the method by which it is carried out, the following examples are provided. These examples are intended only to be illustrative and are not intended to delimit the invention in any manner.

EXAMPLE I An approximately 6-inch long quartz ampoule having an ID of about 8 millimeters and an OD of about 12 millimeters is first washed and then etched in a conventional manner by rinsing the ampoule with an etch solution. After being so treated, the ampoule is rinsed with deionized water followed by distilled water. The ampoule is then vacuum fired and placed in a glove box containing a nitrogen atmosphere. In the glove box, the ampoule is loaded with 7.7259 grams of mercury, 1.1433 grams of cadmium, and 6.1808 grams of tellurium. After loading, the ampoule is evacuated to a residual pressure of 3 times 10' Torr, sealed with a hydrogen and oxygen torch, inserted into a fire brick insulator similar to that shown in FIG. 1, and wrapped with a layer of quartz wool. The ampoule is then positioned in a conventional rocking furnace. The bottom end of the furnace is provided with a quartz wool pack around which a stainless steel tube having an ID of approximately 5 millimeters. The tip of the tube is positioned approximately one-half inch from the bottom of the ampoule. The rocking furnace is raised to a temperature of about 810 C. and rocked for a 24 hour period while the temperature is maintained at that level.

At the end of the 24 hour period, during which the mercury, cadmium and tellurium are maintained in the liquid state to permit reaction between the components to form a homogeneous liquid mixture, the furnace is stopped and the plug at the upper end of the furnace removed. Essentially simultaneously the current used to heat the furnace is cut off and nitrogen is introduced through the stainless steel tube under a pressure of about 6.2 psig. After striking the bottom of the ampoule, the nitrogen circulates around the sides of the ampoule and through the quartz wall layer surrounding the ampoule (layer 26, FIG. 1). The nitrogen exits through the upper end of the furnace. The mercury cadmium telluride is quenched from the liquid to the solid state in approximately 25 seconds.

After the alloy is quenched to the solid state, the ampoule is removed from the furnace and the ingot removed from the ampoule. Thereafter the ingot is annealed by conventional methods. The ingot over its length of approximately 4 inches is sliced into 20 substantially equal slices along planes perpendicular to the axis of the ingot. After analysis of the individual slices the ingot is found to be substantially homogeneous. About 75% of the ingot is found to be usable.

EXAMPLE 2 The procedure of Example 1 is repeated except that no thermal insulator is utilized for the vapor space. After quenching with a nitrogen stream the solidified alloy is removed from the ampoule, annealed and sliced into 20 substantially equal slices along its length. Upon examination of the slices, evidence of pitting and blow holes are observed on the slices which were originally near the vapor space in the ampoule. The occurrence of the blow holes and pitting, of course, cuts down the usable yield of alloy from the ingot. The usable portion of the ingot is less than that obtained with the ingot of Example 1.

EXAMPLE 3 The heating procedure of Example I is followed except that l0.2608 grams of mercury, 1.4736 grams of cadmium and 8.2656 grams of tellurium are utilized as starting materials. The cooling gas tube is removed from the furnace and replaced by a quartz wool plug. After maintaining the temperature of the furnace at 810 C. for a 24 hour period, the insulator and the ampoule containing molten alloy are removed from the furnace by grasping a wire handle similar to that illustrated in FIG. 2. Immediately thereafter the tip of the ampoule containing the molten alloy is immersed to a depth of about 1 centimeter in a quenching oil. The time required for solidification of the alloy is less than 20 seconds. Thereafter, the ingot is removed from the ampoule, annealed and sliced into two portions along its axis. No evidence of piping or blow holes is discovered. The ingot is substantially homogeneous throughout its entire length. Substantially the entire ingot is usable for the production of electromagnetic radiation detectors.

The foregoing specification discloses a preferred method for producing homogeneous alloy ingots, specifically those composed of cadmium, mercury and tellurium, which are not piped or pitted and substantially all of which can be utilized for the desired use, primarily for electromagnetic radiation detectors. This invention, defined in the appended claims, provides a method by which substantially all of the originally formulated components making up the alloy are converted to a homogeneous solid alloy ingot substantially all of which can be utilized in production of electromagnetic radiation detectors.

What is claimed is:

1. In a method of producing a mercury cadmium telluride alloy by sealing the desired proportions of mercury cadmium and tellurium in an ampoule sufficiently large to include a vapor space and heating the ampoule and contents in a furnace to at least a temperature at which the components will form a homogeneous liquid mixture, the improvement comprising:

thermally insulating the portion of the ampoule containing the vapor space, removing the ampoule from the furnace, quenching the alloy from the liquid to the solid state by cooling at least a portion of the space occupied by the alloy.

2. The method of claim 1 wherein the quenching is accomplished by immersing at least a portion of the space of the ampoule occupied by the alloy in a quenching bath.

3. The method of claim 2 wherein only a portion of the space of the ampoule occupied by the alloy is immersed in a quenching bath.

4. The method of claim 3 wherein the quenching bath is maintained at approximately room temperature;

5. The method of claim 4 wherein the alloy is solidified in less than 20 seconds.

6. The method of claim 5 wherein the quenching bath is composed of a quenching oil.

7. In a method of producing an alloy wherein one of the components has a relatively high vapor pressure by sealing the desired alloy components in an ampoule sufficiently large to include a vapor space therein, and heating the ampoule and contents to a temperature sufficiently high to form a homogeneous liquid mixture of said components, the improvement comprising:

quenching the alloy from the liquid to the solid state by selectively cooling that portion of the ampoule containing the alloy, while maintaining the temperature of the remaining portion of the ampoule sufficiently high to prevent flashing of said high-vapor-pressure component from within the molten alloy during said quenching.

8. A method as defined by claim 7 wherein said selective cooling is achieved by thermally insulating that portion of the ampoule containing the vapor space and immersing that portion of the ampoule occupied by the alloy in a quenching bath.

9. A method as defined by claim 7 wherein the high vapor pressure component of said alloy is mercury.

10. The method of claim 7 wherein said alloy comprises at least one Group II element and at least one Group Vl element. 

2. The method of claim 1 wherein the quenching is accomplished by immersing at least a portion of the space of the ampoule occupied by the alloy in a quenching bath.
 3. The method of claim 2 wherein only a portion of the space of the ampoule occupied by the alloy is immersed in a quenching bath.
 4. The method of claim 3 wherein the quenching Bath is maintained at approximately room temperature.
 5. The method of claim 4 wherein the alloy is solidified in less than 20 seconds.
 6. The method of claim 5 wherein the quenching bath is composed of a quenching oil.
 7. In a method of producing an alloy wherein one of the components has a relatively high vapor pressure by sealing the desired alloy components in an ampoule sufficiently large to include a vapor space therein, and heating the ampoule and contents to a temperature sufficiently high to form a homogeneous liquid mixture of said components, the improvement comprising: quenching the alloy from the liquid to the solid state by selectively cooling that portion of the ampoule containing the alloy, while maintaining the temperature of the remaining portion of the ampoule sufficiently high to prevent flashing of said high-vapor-pressure component from within the molten alloy during said quenching.
 8. A method as defined by claim 7 wherein said selective cooling is achieved by thermally insulating that portion of the ampoule containing the vapor space and immersing that portion of the ampoule occupied by the alloy in a quenching bath.
 9. A method as defined by claim 7 wherein the high vapor pressure component of said alloy is mercury.
 10. The method of claim 7 wherein said alloy comprises at least one Group II element and at least one Group VI element. 